Annealing is a thermal process to expose a target material to a high temperature for a long time and then cool it down slowly. Traditional furnace annealing is incapable of thoroughly eliminating defects in crystals even at a temperature up to 1100° C., while semiconductor laser annealing allows relative thorough removal of such defects. Laser light wavelengths that can be used in semiconductor laser annealing span a wide spectrum, from the ultraviolet (UV) region to the infrared (IR) region, and there have been developed a variety of semiconductor laser annealing methods, such as the single-pulse laser annealing disclosed in U.S. Pat. No. 6,336,308 (issued on Jan. 8, 2002) and U.S. Pat. No. 7,365,285 (issued on Apr. 29, 2008), high-frequency Q-switch pulsed laser annealing, scanning continuous-wave laser annealing, same-wavelength double-pulse scanning laser annealing, dual-wavelength double-pulse scanning laser annealing, etc.
Double-pulse laser annealing is superior to single-pulse laser annealing in performance in terms of implanted ion activation, as described in the references, “Silicon Laser Annealing by a Two-Pulse Laser System with Variable Pulse Offsets” by V. Gonfa et al., “Laser Annealing of Double Implanted Layers for IGBT Power Devices”, by Clement Sabatier et al., and “UL Dual Beam Laser Spike Annealing Technology”. There are two types of double-pulse laser annealing. One is to irradiate two laser pulses of the same wavelength onto a silicon surface at different instants of time (See, e.g., V. Gonfa et al., “Silicon Laser Annealing by a Two-Pulse Laser System with Variable Pulse Offsets”, and Clement Sabatier et al., “Laser Annealing of Double Implanted Layers for IGBT Power Devices”). The other is to first preheat the silicon surface with long-wavelength continuous or pulsed laser radiation and then anneal the silicon surface with short-wavelength laser radiation (See, e.g., “UL Dual Beam Laser Spike Annealing Technology”). At present, laser annealing apparatuses that employ the double-pulse schemes have started to be used in the fields of insulated-gate bipolar transistors (IGBTs), thin-film transistors (TFTs) and the like.
As disclosed in the prior art of “Liquid Phase Reflectivity under Conditions of Laser Induced Silicon Melting”, in the course of laser annealing, after the silicon surface is melted, the formed liquid silicon has a reflectivity two times that of solid silicon, thus causing an additional part of the laser energy to be reflected off rather than absorbed in the silicon material. This will reduce the laser energy utilization efficiency and hence harming the performance of the annealing process.
This problem is commonly associated with conventional single- and double-pulse laser annealing methods. If elongating the time delay between the two laser pulses, this may lead to an overly low temperature across a certain time frame that fails to meet the annealing requirement (i.e., the temperature should be kept over 1300° C. for at least 50 ns). Further, in some applications (e.g., the annealing described in the prior art reference of “Sub-Melt Laser Annealing Followed by Low-Temperature RTP for Minimized Diffusion” by S. B. Felch, D. F. Downey, E. A. Arevalo, et al. of the Varian Semiconductor Equipment Associates, Inc., 811 Hansen Way, Palo Alto, Calif., 94303-0750, USA, and the solar energy annealing described in the prior art reference of “Pulsed Laser Annealing and Rapid Thermal Annealing of Copper-Indium-Gallium-Diselenide-Based Thin-Film Solar Cells”), a desirable annealing effect is obtainable at a temperature not exceeding the silicon melting point of 1414° C.
No matter a single- or double-pulse laser annealing method is used, when the temperature exceeds the silicon melting point, the laser energy absorption will drop by half and the utilization efficiency will decrease correspondingly. In addition, it is possible for the two annealing methods described above to fail to maintain a desired annealing temperature (e.g., 1100° C., at which an activation efficiency of 90% is achievable) for a sufficiently long period of time (e.g., ≧100 ns) and to thus have an undesirable annealing effect. Therefore, improvements in energy utilization efficiency and annealing effect are particularly desirable in the field of laser annealing.